A cooling fan mainly includes a stator and a rotor. The rotor has a rotor shaft rotatably received in a bearing provided on the stator, and is therefore rotatably supported on the stator to rotate freely.
For the rotor shaft to always smoothly rotate in the bearing, lubricating oil is usually applied between the rotor shaft and the bearing. However, when the cooling fan operates, there is electric current flowing through the windings on the stator to thereby produce heat. The produced heat will transfer from silicon steel plates on the stator and a bearing seat to the bearing, so that the bearing always works under a relatively high temperature, which would cause reduced viscosity and accordingly, increased fluidity of the lubricating oil or lubricating grease in the bearing, resulting in reduced load capacity of the cooling fan as well as quick loss of the lubricating oil from between the bearing and the rotor shaft. Without sufficient lubricating oil for lubricating and cooling, the bearing and the rotor shaft are in frictional contact with each other to cause shortened service life of the rotor shaft and the bearing, and accordingly, of the cooling fan. In conclusion, the conventional cooling fan has the following disadvantages: (1) having excessively high working temperature; (2) having shortened service life; and (3) having poor heat insulation effect.